I have a lot of images on my site that are submitted by visitors on the site. Is there anything I can do to them, like change their alt tag, to optimize them?
If I were you I would host various sizes of the same image, be sure to alt tag, title tag, and link to you homepage index via text link to make the images relative. Once done you will be seeing the Google Bot searching your images.
The images are actually on my homepage already, but in small posts submitted by other people. Should I still link them to my homepage? Also, will this increase my PR or rankings?
All images that are on the homepage or siteindex will be found as more relative. Do you use a sitemap.xml or HTML site for indexing?
I think I have a sitemap. I am based off of Wordpress, and I actually tried setting it up today, but I am not sure if it is up correctly and live. Is there a way I can check?
Try this from Wordpress: wordpress.org/extend/plugins/google-sitemap-generator I believe you do have xml default...but not to sure. Yoast Plugin is awesome for SEO on Wordpress BTW.