When using a template which has separate definitions for landscape and portrait orientated photos (vertical and horizontal tables) like Switchy for example, you will encounter problems with the Secret Image Addon.
You have one table for horizontal view and one for vertical view.
Depending on which orientation the image has, the one table is set invisible and the other visible (by css).
Now it happens that the image with name "mainimage" is in the page twice, even if not visible.
Firefox changes the image in the first occurance (which is the horizontal view) but not again in the second.
InternetExplorer seems to be more tolerant in having the same name twice.
Here is a modified version of Secret Image Addon for those who are using a template which has the image layout part twice, depending on image orientation:
Please use these Addon Tags instead of <ATTRIBUTE_NAME> and <SECRET_LINK>:
<ATTRIBUTE_NAME1> and <SECRET_LINK1> in the horizontal table
<ATTRIBUTE_NAME2> and <SECRET_LINK2> in the vertical table
Have fun with it!