From Add - Converter - ColorRamp
Plug in the connection from the Image Sequencer node to the input on the ColorRamp node (called Fac).
The ColorRamp converter is similar to using Levels in Gimp, you adjust contrast and dark and bright levels. So in this example case we need to raise the black in the Stickman image. (It's a thin grey pencil drawing.) You may not need this in your images, or your values will need to be different.
Again from Add - Matte - Color Key
Plug in Color Ramp Image Output into Color Key Image Socket.
Leave the color as pure white for now. If your drawing or image has a different background color, you would set it to that color and skip the ColorRamp and perhaps try Chroma Key or Keying node instead of Color Key, but for this example we assume it is a black and white drawing.
Plug in Color Ramp Image Output into Color Key Image Socket
Now plug in Color Key Image Output both into Viewer Image Socket AND into Composite Image Socket.
To remove the white background, adjust V, values on Color Key. (If it was another colour you could try adjust the H and S (hue and saturation) values too.
Try to raise the black color stop (the lower "arrow") on Color Ramp by dragging it towards the white. This may help increase contrast and make your image stand out more.
Now you probably need to adjust Values on Color Key again.
Experiment with these values on ColorRamp and Color Key nodes until it fits your image.
Zoom in and out on the image to check for best results.
To export your image sequence, click on Render from the menu on the top right and choose Render Animation (if you choose Render Image you will only get one Image.)