I can't seem to find any details about the Picture widget. What format & size work best ?
There is no documentation but it's in the source code:
if ($_FILES['pictfile']['size'] > (10 * 1024 * 1024)) {
$input_errors[] = gettext("The image file is too large. Please upload something smaller than 10MB.");
So file size is limited to 10 MB. I don't see a pixel limit.
I don't know if transparency is supported but I would suggest .png or .gif format for logos and .jpg for photos.