Has anyone seen SPAM issues using certain image files over others? I usually use jpg's in mine, but has anyone seen problems with .gif, .png, etc?
I've certainly never heard of an image file type causing an email to be blocked as spam. JPEG and GIF's are definitely fine, but I wouldn't be sure about PNG support in some older email clients that use an older rendering engine. having said that, this is only a potential rendering issue, and isn't at all related to spam filtering.
To be safe, I'd recommend sticking to GIF and JPEG, but rest assured this won't impact your chances of being filter, this will.