I did a small test on images about a month ago regarding images as I said I was sure google was reading the EXIF data within images to help them index.
So below are my findings.
I posted a new image on my website on a not very popular page.
I put some text just under the image,
I called the image something different
I placed a different ALT tag on the image.
I added EXIF data and put a description into that.
If I searched for the ALT text the results are #2
If I search for the text under the image results are #3
If I search for the image name its not listed (without the .jpg)
If I search for the description I put in the EXIF data it is not listed
So the only conclusion you can come from this test is that the alt text is the most important and slightly behind that is the text near the image.
There is little you can do to the image or embed into it that will effect your position much.
In your case where your posting the image onto a forum its the text within the forum and around the image that is making the image rank higher and not the image itself
Last edited by Chedders; Dec 13th, 2012 at 05:40 PM.
Good SERPS is great, Traffic however is even better