How to delete a photo from Forum?

Not true.  We post a "link" only to the DPR galleries.  For images uploaded from a computer or from a URL the image is not linked to the original source, but copied to DPR.   (See below for the significance.)

Users at this time can only link to gallery images.  The gallery image can be removed by the user at any time, which will also remove the image from any forum article which refernences it with a link.

However, users cannot delete the images uploaded from a URL or from a computer, and therefore they cannot remove those images from a posted forum article unless it is done immediately after posting (users have a time limit and are limited in the number of edits).

Moderators can edit a article any number of times and after any length of time.  Entire articles can be removed, or just the image edited out of the article.

Note that when a moderator deletes an article it also deletes all articles which have replied to that article.  We might see that change at some point in the future, but I don't know that DPR admin's have promised that will happen.

A future change which has been specifically promised "soon" is the ability to actually link to images via a URL to another site.  That could be to any of the various image display sites or it can be a link to a user's personal website.  The distinction between linking to a DPR gallery image or uploading to DPR and making a link to another site is the ability to then control what is at the end of the link.  With the DPR gallery as far as I know you cannot replace an image with a different image that will have the same DPR link.  All that can be done is to remove it.  With a real URL link the image can be deleted, but it can also simply be replaced with a new image that has the same name.  The new image will from that point on be the one seen via a link inserted into a forum message.

But see the above for the significant difference in what happens when those to different methods for inserting images into a message are used.

Note specifically that the three icons in the message editor that are used for inserting images do NOT use the term "link", they all say "insert".