I have installed in joomla 1.7 the plg_jw_sigpro-v2.5.6_j1.5-1.7.
After adding a lot of galleries in Single Thyumbnail display mode on  a page, when I exceed a number of them, I get a blank page.
See here:
www.eurolinea.gr/home/index.php/2011-12-14-14-30-36/2011-12-14-14-51-51
for example, if I add another series Single Thumbnail galleries, the page doesn't show.

the php memmory in the php.ini is 128 mb

Your issue has been already answered in the SIG PRO section where it belongs.

this???

forum.joomlaworks.net/simple-image-gallery-pro-%28plugin%29/%28troubleshooting%29-gallery-thumbnails-don%27t-display-at-all-%28or-display-%27blank%27%29/

No! This:
forum.joomlaworks.net/simple-image-gallery-pro-%28plugin%29/blank-page-and-out-of-memory-when-litespeed-is-running-on-server/msg65825/#msg65825
and this:
forum.joomlaworks.net/simple-image-gallery-pro-%28plugin%29/blank-screen-when-loading-thumbnails/msg51781/#msg51781