phpBB automatically creates alt tags based on an image file name. Example this-is-my-image.jpg would have alt text as "this-is-my-image". But i don't think Google will accept this at correct format and thus will not index the image in Google image search. More on that here. Can a phpBB dev confirm this and help explain why even create that type of alt text option? I upload my images using dash format "this-is-my-image" and would rather phpBB convert the image filename to correct alt text without the dashes.
I am guessing anyone that wants alt image text to appear correctly and be set must use an extension such as the ones below
viewtopic.php?f=676&t=2542461
Let me know if I'm correct please.