Instructions
Have Your Sim Create Artwork
Click the "Shop" icon and purchase an easel.
Click on the easel and choose "Paint" from the options list.
Keep painting until the picture is finished. This may take several sessions, but your mood will increase along with your painting skill. When your Sim stops painting before you give him another command, or goes to the easel and then leaves, the painting is finished.
Click on the painting, choose "Hang" and deposit the painting on an empty spot on the wall.
Use Third-Party Software to Edit Objects
Download and install object editor software from the official Sims website (thesims3.com), which will allow you to create your own objects from scratch or edit objects already present in the Sims Shop.
Look through the Sims shop for a piece of art you would like to modify. Open the software and choose the .iff file for that object from the Sims object file directory. The software will automatically detect the directory.
Click "Upload File" and choose a photograph or other image document from your computer. Drag the photograph to the frame, then click on a corner and drag the photo to the appropriate size.
Click "Save," rename the file and save it to the same file directory as the original object.
Open your Sims game, click "Shop" and purchase the picture. Move it to the spot on the wall where you would like to hang it and click the mouse button to hang it.