I have heard this too (too much HTML) but as of right now I don't believe it to be true...I did an experiment last week...I listed some items directly thru ebay (plain and boring) and some thru Auctiva (templates and the works). I actually had more sales with the Auctiva listings! Could be coincidence, it's impossible to tell, but these were all my best selling items. I had also heard about the scrolling gallery (which I always loved) so I DID disable that but I can honestly say it hasn't made one bit of difference. I'm enabling it again today.
I have found after all these years that most people don't read the listing anyway so they never scroll down far enough to even click to supersize a pic (I hate to say it but I'm guilty of it myself, lol). You have to always remember that we sellers know INFINITELY more about selling on ebay than the ebay reps do, take what they say with a grain of salt.
I'm not super computer literate, but I don't think Auctiva's images qualify as 'embedded'. I think he meant adding a huge pic to the body of the listing through Photobucket, or etc...so until I see REAL evidence that Auctiva templates or whatever is hurting my sales I will keep on using them.
This is all just my own opinion btw