Hello Pætur Joensen,

Sorry for late reply to this,
Yes, you can upload the image file in Easyblog media manager, then click on the "insert as gallery" button, it will display on your blog entry page.
Once you click one of the image, it will show the pop-up. Check my screenshot below.

Yes, you can upload the category image from your frontpage or backend.
backend > Easyblog > category > select your category > avatar
frontend > setting > category > edit category > avatar
Hope this help.