You can always set it to 'diabled' so it won't show any signatures. I browse the forums that way and it the pages seem to flow in smoothly without any lag.
Its the same with the Avatars.
As for loading in a pictures, I dont have one and I haven't tried to load one, but I think it may be a file size restriction or perhaps the image is just too big in general.
EDIT: I just looked it up and this are the rules for loading pictures from your computer
Note: The maximum size of your custom image is 500 by 100 pixels or 19.5 KB (whichever is smaller).
Which means the files resolution cannot be bigger that 500 X 100 pixels. If you are computer illiterate then I suggest just right-clicking the file you are trying to load and in the 'general' panel it should display the resolution.(On XP it also displays if you hover your mouse over the file). Similarly the file size is also restricted to 19.5 KB which again can be found under the file properties.
If you're image does happen to cross the boundaries, then I suggest loading it up in paint and cutting off edges or opening up photoshop and doing an 'image resize' there.
EDIT #2: I also forgot to mention that there is an alternative. You can always use image hosting sites to load up your image and linkit here. For example, if you go to www.imageshack.us
you can upload your image there and just copy and paste the link in the appropriate place in your 'edit signature panel'