There exist two image types that display on the Tabletop: Background Images and Tokens.
Background images are any graphics that you do not plan to move or directly interact with while playing your game. This would be images such as a game board or the terain of a battlemap. These images are best kept on the Map & Backgound Layer.
Tokens are any image that you plan to manipulate on the Tabletop. This would includes images of RPG minis, board game components, currency chips, etc. If you would like an image to be treated as a token, it must be placed on either the Objects & Tokens Layer or the GM Info Overlay Layer.
Placing Images Onto the Tabletop
Roll20 currently accepts the following image file types: JPG, PNG, and GIF. Files with transparent elements will maintain their transparency on the Roll20 Tabletop.
Keep in mind that currently JPEG is not a supported file type. If you are having difficulty uploading a JPEG you can easily change it to JPG via most image editing programs.
Image Insertion Methods
There are two ways to place images onto the Tabletop: Direct Upload or an Image Search
To use images from your own personal files, click on an image from your computer files and drag it directly onto The Tabletop. This will initiate an upload of that image to Your Art Library. When the upload is complete, it'll display the image on The Tabletop.
If you would prefer to batch upload a series of images at once, you can do this from the Your Art Library window that you can access from the Art Library tab of the Sidebar. There is a 10 files/5 MB file size per batch limitation to batch uploads.
This method is accomplished by going to the Art Library tab in the Sidebar and running a search for a desired art element. When you find an image that you wish to use, click the thumbnail from the search and drag it onto The Tabletop.
Snap to Grid
Anytime the grid is enabled in Page Settings, Snap To Grid is enforced on the Tabletop. Snapping affects all transformations that can be made on your Background Images as well as Tokens. Hold down the ALT Key while moving, rotating, or scaling to ignore snapping.
When selected, a handle will stick out from the top of the image. This is the rotation gizmo. Click the end of the handle for rotation. While Snap To Grid is enabled, an image rotates in increments of 45° for square grids and 30° for hex grids.
You can also rotate a selected token by holding down the E key while scrolling your mouse wheel. This will rotate snap it to 45° or 30° increments. If you also hold down the Alt key, the token will rotate in much finer increments.
The Scale transformation tool is available only to GMs within a game. Players cannot access this functionality regardless of their control of a token.
The Scale gizmo is the bounding box that surrounds a selected image. If you click and drag the nodes on either the top or bottom of the bounding box, you will vertically scale the image. The nodes on either side of the image will horizontally scale it. Clicking and dragging any of the corner nodes will universally scale both horizontally and vertically simultaneously. Images scale from the center of the image. While Snap To Grid is enabled, an image will scale to the nearest grid line in either direction.
If you don't like how you transformed your images, using the keyboard shortcut Ctrl/Cmd Key + Z will undo the last action.
You can access more advanced features by right-clicking on a graphic to bring up a graphics menu and selecting Advanced.
Grouping and Ungrouping
While a group of graphics is selected, clicking on the Group option from the Advanced menu will bind the graphics together into a singular movable block. You can remove grouping by selecting Ungroup.
If you wish to move the group as a whole, click once to select the group and then click and drag. If you wish to move a single graphic independently of the group, click on it and drag it normally. The group will remain highlighted, but that token will move without moving any other members of the group.
These two menu options will flip the graphic horizontally or vertically.
The As Drawing menu option changes a graphic on the Token & Objects and GM layers by removing the suite of Token features normally attributed to it. It will ignore grid snapping in this mode and can also be selected along with other hand drawings by holding down the Alt key while selecting items on the Tabletop.
If there's a specific pixel/unit height and width you want to force a particular graphic to conform, selecting Set Dimensions brings up a new window where you can input the desired dimensions.