I cannot upload a lookbook image when creating a lookbook.

I also get an error in the system.log:

[2018-01-10 11:24:14] main.CRITICAL: Unable to resolve the source file for 'adminhtml/Magento/backend/en_US/MGS_Lookbook/images/btn_over_bg.gif' [] []

I get the following error in the report:

{"0":"Notice: A non well formed numeric value encountered in \/var\/www\/html\/magento2\/app\/code\/MGS\/Lookbook\/Controller\/Adminhtml\/Lookbook\/Upload.php on line 124","1":"#0 \/var\/www\/html\/magento2\/app\/code\/MGS\/Lookbook\/Controller\/Adminhtml\/Lookbook\/Upload.php(124): Magento\\Framework\\App\\ErrorHandler->handler(8, 'A non well form...', '\/var\/www\/html\/m...', 124, Array)\n#1 \/var\/www\/html\/magento2\/app\/code\/MGS\/Lookbook\/Controller\/Adminhtml\/Lookbook\/Upload.php(104): MGS\\Lookbook\\Controller\\Adminhtml\\Lookbook\\Upload->toBytes('8M')\n#2 \/var\/www\/html\/magento2\/generated\/code\/MGS\/Lookbook\/Controller\/Adminhtml\/Lookbook\/Upload\/Interceptor.php(50): MGS\\Lookbook\\Controller\\Adminhtml\\Lookbook\\Upload->checkServerSettings()\n#3 \/var\/www\/html\/magento2\/app\/code\/MGS\/Lookbook\/Controller\/Adminhtml\/Lookbook\/Upload.php(77): MGS\\Lookbook\\Controller\\Adminhtml\\Lookbook\\Upload\\Interceptor->checkServerSettings()\n#4 \/var\/www\/html\/magento2\/generated\/code\/MGS\/Lookbook\/Controller\/Adminhtml\/Lookbook\/Upload\/Interceptor.php(24): MGS\\Lookbook\\Controller\\Adminhtml\\Lookbook\\Upload->execute()\n#5 \/var\/www\/html\/magento2\/vendor\/magento\/framework\/App\/Action\/Action.php(107): MGS\\Lookbook\\Controller\\Adminhtml\\Lookbook\\Upload\\Interceptor->execute()\n#6 \/var\/www\/html\/magento2\/vendor\/magento\/module-backend\/App\/AbstractAction.php(229): Magento\\Framework\\App\\Action\\Action->dispatch(Object(Magento\\Framework\\App\\Request\\Http))\n#7 \/var\/www\/html\/magento2\/vendor\/magento\/framework\/Interception\/Interceptor.php(58): Magento\\Backend\\App\\AbstractAction->dispatch(Object(Magento\\Framework\\App\\Request\\Http))\n#8 \/var\/www\/html\/magento2\/vendor\/magento\/framework\/Interception\/Interceptor.php(138): MGS\\Lookbook\\Controller\\Adminhtml\\Lookbook\\Upload\\Interceptor->___callParent('dispatch', Array)\n#9 \/var\/www\/html\/magento2\/vendor\/magento\/module-backend\/App\/Action\/Plugin\/Authentication.php(143): MGS\\Lookbook\\Controller\\Adminhtml\\Lookbook\\Upload\\Interceptor->Magento\\Framework\\Interception\\{closure}(Object(Magento\\Framework\\App\\Request\\Http))\n#10 \/var\/www\/html\/magento2\/vendor\/magento\/framework\/Interception\/Interceptor.php(135): Magento\\Backend\\App\\Action\\Plugin\\Authentication->aroundDispatch(Object(MGS\\Lookbook\\Controller\\Adminhtml\\Lookbook\\Upload\\Interceptor), Object(Closure), Object(Magento\\Framework\\App\\Request\\Http))\n#11 \/var\/www\/html\/magento2\/vendor\/magento\/framework\/Interception\/Interceptor.php(153): MGS\\Lookbook\\Controller\\Adminhtml\\Lookbook\\Upload\\Interceptor->Magento\\Framework\\Interception\\{closure}(Object(Magento\\Framework\\App\\Request\\Http))\n#12 \/var\/www\/html\/magento2\/generated\/code\/MGS\/Lookbook\/Controller\/Adminhtml\/Lookbook\/Upload\/Interceptor.php(78): MGS\\Lookbook\\Controller\\Adminhtml\\Lookbook\\Upload\\Interceptor->___callPlugins('dispatch', Array, NULL)\n#13 \/var\/www\/html\/magento2\/vendor\/magento\/framework\/App\/FrontController.php(55): MGS\\Lookbook\\Controller\\Adminhtml\\Lookbook\\Upload\\Interceptor->dispatch(Object(Magento\\Framework\\App\\Request\\Http))\n#14 \/var\/www\/html\/magento2\/vendor\/magento\/framework\/Interception\/Interceptor.php(58): Magento\\Framework\\App\\FrontController->dispatch(Object(Magento\\Framework\\App\\Request\\Http))\n#15 \/var\/www\/html\/magento2\/vendor\/magento\/framework\/Interception\/Interceptor.php(138): Magento\\Framework\\App\\FrontController\\Interceptor->___callParent('dispatch', Array)\n#16 \/var\/www\/html\/magento2\/app\/code\/Infortis\/Cgen\/Plugin\/Magento\/Framework\/App\/FrontController.php(32): Magento\\Framework\\App\\FrontController\\Interceptor->Magento\\Framework\\Interception\\{closure}(Object(Magento\\Framework\\App\\Request\\Http))\n#17 \/var\/www\/html\/magento2\/vendor\/magento\/framework\/Interception\/Interceptor.php(135): Infortis\\Cgen\\Plugin\\Magento\\Framework\\App\\FrontController->aroundDispatch(Object(Magento\\Framework\\App\\FrontController\\Interceptor), Object(Closure), Object(Magento\\Framework\\App\\Request\\Http))\n#18 \/var\/www\/html\/magento2\/vendor\/magento\/framework\/Interception\/Interceptor.php(153): Magento\\Framework\\App\\FrontController\\Interceptor->Magento\\Framework\\Interception\\{closure}(Object(Magento\\Framework\\App\\Request\\Http))\n#19 \/var\/www\/html\/magento2\/generated\/code\/Magento\/Framework\/App\/FrontController\/Interceptor.php(26): Magento\\Framework\\App\\FrontController\\Interceptor->___callPlugins('dispatch', Array, Array)\n#20 \/var\/www\/html\/magento2\/vendor\/magento\/framework\/App\/Http.php(135): Magento\\Framework\\App\\FrontController\\Interceptor->dispatch(Object(Magento\\Framework\\App\\Request\\Http))\n#21 \/var\/www\/html\/magento2\/generated\/code\/Magento\/Framework\/App\/Http\/Interceptor.php(24): Magento\\Framework\\App\\Http->launch()\n#22 \/var\/www\/html\/magento2\/vendor\/magento\/framework\/App\/Bootstrap.php(256): Magento\\Framework\\App\\Http\\Interceptor->launch()\n#23 \/var\/www\/html\/magento2\/index.php(39): Magento\\Framework\\App\\Bootstrap->run(Object(Magento\\Framework\\App\\Http\\Interceptor))\n#24 {main}","url":"\/magento2\/admin\/admin\/lookbook\/upload\/key\/0ee49506c0dbdbaa409fdf002a84a4b5c8953636c4dee212008078d354d84f64\/?form_key=n1Qdy4fxzMw2kjIE&qqfile=lookbook.png","script_name":"\/magento2\/index.php"}

the problem was a missconfig in php - got it fixed