Timthumb special characters

I have issues with timthumb,

if i try to use a theme which uses Timthumb to create thumbnails the pictures will not be shown. I found out that this happens only if the picture includes a “_” like IMG_1234.jpg in the filename. But i have 1000 of pictures with underlines in the filename. Its not possible to rename them all.

If i use a highlightet images without a underline the picture path is right like:
http://www.*****.com/wp-content/themes/journalcrunch/timthumb.php?src=http://www.****.com/wp-content/gallery/pictures/img_0543.jpg&h=90&w=255&zc=1

if it contains a underline the picture path doesnt contain the path to the original picture:
http://www.*****.com/wp-content/themes/journalcrunch/timthumb.php?src=h=90&w=255&zc=1

Is there a way to get Timthumb running with this filenames?

Regards Marc