“On” and “Off” radio buttons (upper left) :  turn random startup picture choosing on and off.
“Import…” :  imports all PICT resources from any file.
“On” and “Off” buttons (lower right) :  enable/disable the selected pictures for random choosing.
“New” :  create new picture. Pastes clipboard if available, else uses default picture.
“Delete” : permanently delete all selected pictures.
Control-number  sets the weight of all selected pictures to number.

Select pictures by clicking on them in the picture list (lower left). Shift-click or Command-click to extend the selection.

The picture list shows from left to right:  a number telling the picture´s relative weight or nothing if the picture is disabled; a bullet for the picture to show up at next startup; the name of the picture.

The following only applies if exactly one picture is selected in the picture list :

“Set” makes the selected picture the one showing up at next startup.
The scrollbar above the picture list is used to change a picture´s weight if it is enabled.
The text field above the scrollbar (and below the “Import…” button) is used to edit the picture name. It features Cut, Copy, Paste, Clear and Undo. The name is accepted with the Return or Enter key or when an other picture is selected. Editing the name is possible only if the name field is highlited (or the cursor blinks in it).

In the upper left there is the picture preview showing the currently selected picture. If it is highlighted (a fat frame around it in your hilite color), Copy and Paste is supported.
Above the preview the picture size in pixels (horz. by vert.) is shown. For standard sizes their denomination (e.g. 13" RGB) is also shown.
The “Full Size” check box toggles between the “full size” (shows picture as big as possible) and “relative size” (shows picture in relation to your screen) preview modes.
Double-clicking the picture preview shows the picture in its real size as it would appear at startup. Click anywhere or type any key to come back.
The Tab key toggles between selecting the preview and the name.

For further details please refer to the rest of this documentation.