Do you mean you want to set the tilemap's tilesheet image from any other image? If you have the URL of that image (ie: if it's a project file, or a file hosted and accessible from elsewhere) you can use the 'Load image from URL' action on the Tilemap.
Do you mean you want to be able to import an image at runtime and draw the tile with one pixel per tile? In which case you need to load the image into a sprite, draw the sprite at [0,0], then run a 2D loop that will check every cell in your array. For example:
For "x" from 0 to ( Tilemap.Width / Tilemap.Tilewidth )
For "y" from 0 to ( Tilemap.Width / Tilemap.Tilewidth )
Gives you a loop which iterates over every cell in the tilemap. You'll need a way of indexing the tiles to pixel colours, but the pixel colour can be determined using this lovely line from
var pixel=ctx.getImageData("&loopindex( "x" )&","&loopindex( "y" )&",1,1);[/p]