Well, the drop-down list is easy.... as for positioning, you could create a class that aligns it so that it's absolutely positioned:
img.top-left {
position:absolute;
top:0;
left:0;
}
img.top-right {
position:absolute;
top:0;
right:0;
}
img.bot-left {
position:absolute;
bottom:0;
left:0;
}
img.bot-right {
position:absolute;
bottom:0;
right:0;
}
Then just do a regular file upload (search the boards) and you can use the image functions to get the image dimensions. Compare them with what you expect. Then you can (1) resize the image to meet your needs or (2) deny the upload and tell them to resize it themselves. There are scripts on this forum for doing the resizing.