None of the answers so far answer your question about the impacts/benefits to SEO, so here goes:
- The filetype used to make the "button" (jpeg, gif, png etc.) makes no material difference to your SEO
- Using inline images (
img) requires the use of
alt attributes, which would be used as the anchor text for your linked button
- CSS background images, or image replacement techniques can also be used to replace the anchor text with an image. If done correctly (there's lots of good resources on this), there will be no negative SEO impact.
Consider this however, one of the critical elements for mobile performance is speed (now a ranking factor) - so you should consider the need to add additional HTTP requests for those navigation images, rather than styling with CSS.