Dave, from the SEO perspective the title attribute is not necessary, as no major SE give them any weight.
If you want to make sure that the images descriptions show up in Firefox browsers, the you may use the title attribute in the image source, but if the images are linked, you better add them in the anchor hyperlink reference.

I think these two tutorials can guide you about the appropriate implementation of the the images descriptions for SEO, accessibility and overall web style:

Alt Attribute & Search Engine Optimization - SEO Workers
Too much accessibility - good intentions, badly implemented / Public Sector Forums / 8 August 2007 / splintered - random sparks of creative insight / the portfolio and experimental playground of patrick h. lauke aka redux

I hope this helps.

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