load I had this same issue and had to triple-check my php settings until I realized the debug info was telling me it was due to the limit being hard-coded in the file flarum/vendor/flagrow/flarum-ext-image-upload/src/Validators/ImageValidator.php
I was just wrestling with this issue and trying to solve it, so I came back here to report this.
As @luceos noted I think they will fix this in an upcoming version.
I just changed the value in Line #28 to what I needed and it worked.
Line #28 below:
'max:' . app('config')->get('flagrow.image-upload.maxFileSize', 2048)
Note that "2048" there...
EDIT: Just a guess... But I am thinking the original intent was to set the default value to 2048 if the user had not set a value yet. Somehow this overrides the user setting anyway, though...
errr... Or it's just that the modal isn't writing the user setting to this... Or somewhere else.... I Am n00b srry....