We recommend that you always use ALT tags with your images. There are some search engines that record this information in their database. There are some search engines where you can actually search for images - typically using the ALT tags as references for the search. If your web page is composed only of images, the search engine spider will have no idea what your content is about, as it typically will look to the HTML text on your page to try to automatically decipher the web page content. Some spiders will weigh sentences with proper punctuation marks as more relevent than single words.
Some spiders consider text links to be more important.
It is also a good idea to try to raise your HTML text content up on the page as high as possible. Some search engines limits the kilobytes of data that it will scan for its database. Therefore, if the spider doesn't see any HTML text, then it can't figure out your page content. Each search engine has a policy on spamming. Spamming is considered the act of abusing the registration process and the use of some tricks to attempt to raise a web site position.