From Roll20 Wiki
There are three ways you can manipulate your images on Roll20's Tabletop: Move, Rotate and Scale. These transformations are handled by different gizmos that appear over a selected image.
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.
If you don't like how you transformed your images, using the keyboard shortcut Ctrl/Cmd Key + Z will undo the last action.
Movement is achieved by clicking and dragging anywhere inside the selected image. It's best to avoid clicking the outer edges of an image for movement. Doing this might trigger the scaling transformation instead. When Snap to Grid is enabled, an image snaps to the bottom right corner of a grid square and snaps to the center of a grid hex.
Tapping ←, →, ↑ or ↓ arrow keys will move a selected token one grid unit in the specified direction. Holding down the Alt key while using the arrow keys will move a selected token in one pixel increments.
Sometimes while moving a token you may want to indicate how the token moves around corners, or exactly where the token has "stepped" to avoid traps. The Waypoints system allows you to indicate where you're moving to everyone in the game. To use it, just press the "Spacebar" key while you are moving a token. A ruler will appear showing you how far you have moved from your current location. If you want to place a waypoint, press the "Spacebar" again while you are on top of the space you want to move through. A waypoint will appear. You can add as many waypoints as you want showing the path of your token. When you're finished, just release the token like normal at its final destination.
Reviewing Token Movements
You can select any token and press the "Spacebar" key to view the last movement of that token, including waypoints if applicable. All tokens will save their last movement indefinitely, so even if it's been a few weeks since the last time you played, or you're participating in a play-by-post game, you can still see the last movement of any piece on the tabletop.
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 one degree at a time.
Scale (GM Exclusive)
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.
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 moveable block. You can remove grouping by selecting Ungroup.
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.