@Trizek-WMF - the current behavior of the Image field does seem a little bit confusing. It seems that the Image field accepts image files with File: and without it.
Re-checking the steps (and recalling what previous testing has revealed), here is the current behavior:

• the Image field accepts non-existing images and no indication to users will be given.
• saving the non-existing image file won't produce any errors - the succes save message will be shown.
• after "saving" a non-existing image, Community Update module on Special:Homepage won't display an image
• the Image field will be blank

There is another tricky point - adding and saving a valid image after above steps won't be displayed immediately on Special:Homepage, it requires (after getting to Special:Homepage) to refresh the page again. Note - I checked the image File:Wikiconcours Mauve.png is displayed in CommunityUpdate module - enwiki beta Special:Homepage

We should probably check how relevant the guidance changes coming from T362650 and potential upcoming changes for T376466: Lookup: automatic selection when the text typed matches with one result affect this task. (Codex Lookup is the component internally used in our Commons File picker)