AH… assuming then that you have made a costim theme then you juyst need to check this codex page: http://codex.simple-press.com/codex/installation/changes/theme-and-templates-5-4-2/

Look specifically for the new icons part – although it may all be of interest. You can get the default new icon from any of our stock updated themes.

I am having the same problem.  I am using the default 1.2.3 theme, and have not modified any of the icons, etc.  This dead image error started happening after updating to the most recent version of SimplePress.

That is because the new icon comes with the theme. You need to update the Default theme as well – which is now up to 1.2.6. Should show up in your WordPress update page.