If it is a space issue, you will need to upload smaller images to begin with. I'm just looking at an example... gallery 100 has images stored at:
The first image in the list is: _MG_2603.jpg
It was uploaded as a 1.05 MB image. Powergallery created 9 smaller copies of the image to use on the page because you wouldn't want a 1.05MB image used on the web directly. The sum of the sizes of the 9 created images is about 400K... which is less than half the size of the original image.
You can limit the number of images created in the file: /admin/admin_gallery/common/properties.php
deleting the lines with widths and heights will stop the gallery from creating that size image, but some galleries could depend on using that size, so it could prevent some gallery images from displaying properly.
If you removed all but one image, in the example I have shown, it would cut the storage size from 1.4MB to 1.1MB by removing 8 of the 9 image copies.
I think the better solution would be to keep the 9 copies so that all of the galleries would all continue to work, since you don't save much space by deleting them anyway.... but to resize the original image to a smaller size before uploading it, or edit the gallery page /admin_gallery/addimages.php and resize the original uploaded image before processing it.
If you want help resizing the uploaded image on the addimages.php page, this is something we could do in a premier support ticket..