Yes, there are several ways to do that. Hand drawn pictures can use the same technique the great artists of the middle ages used : grid lines. Make copies of your favorite drawing, so you can draw new grid lines on it in pencil. The lines can be 15 degrees apart, like on a globe of the Earth.
You did not post an image of the drawing you made, so I will guess : the drawing is of a globe, with the north pole having accurately drawn land masses that you like. I hope the pole is at the center. Draw lines of longitude that radiate away from the pole, every 15 degrees. 360/15 = 24 so you will draw 24 lines that converge at the pole.
Draw lines of latitude every 15 degrees on the globe picture. There are 12 lines of latitude. Use light blue for all grid lines.When the whole globe is drawn with these lines, there will be 288 sectors with many shapes. The sectors at the poles will be triangles. This may be difficult to do accurately, since the planet you are designing is not drawn on a ball.
Next, a cylindrical projection will be created using those lines of latitude and longitude. Mark a piece of paper with a rectangular grid of 25 vertical lines and 13 horizontal lines. The horizontal lines are not spaced equally, but follow the tangent function. See the link below. This makes 288 rectangles. Hand draw, into each rectangle, the land shape seen on the globe view for the corresponding sector. Use black lines. The triangular sectors at the poles will become rectangles by stretching the land shapes by amounts proporational to the size of the parts of the rectangles to be filled.
By redrawing the 288 sectors, the distortion due to stretching becomes easier to handle with some accuracy. It is easier to handle each rectangle than it is to handle the whole picture at once. This new cylindrical projection can be scanned into a jpeg file, or any format. Use photoshop to make the grid disappear. The light blue can be made to disappear , while black stays.
This new texture should be in a cylindrical projection, and that is what Celestia needs.
Additional information on projections :
http://mathworld.wolfram.com/CylindricalProjection.html
I forgot to mention the tangent function is used to stretch the latitudes.
See the link.
But that transformation is from a sphere to a flat rectangle. I assume you are not starting from a sphere, but from a flat picture of a globe. Please show images of your starting point.
Or even better, get a ball and draw the continents on a ball. Then, accurate projections can be created.
