There are several ways to create 3D models from BMP images. This tutorial explains how to do this by converting a BMP file to an STL file.
The following content was taken from Stratasys' Resources Center
The basic principle is to take a regular 2D image (in BMP format) and translate the image’s gray shades into a 3D height map (Figures 1 and 2). The two-dimensional array (i.e. color) of each element is interpreted as a height value in order to store information on the height of each point. For example, the white points are flat, the gray points are higher and the black points are the highest.
The following steps in this procedure were accomplished using Microsoft® Paint image viewing software and Materialise® Magics STL editing software, and make reference to the menu options in those applications. Other similar software programs may be used to accomplish this procedure.
Step 2: Convert the Image to BMP Format
Open the image in the image viewing software that is capable of converting the file to a bitmap (BMP) format. Then save the image as a bitmap (BMP) file (Figure 6).
Step 3: Create the STL File
Create an STL file from the BMP file by using the Materialise Magics editing program. Click on the Edit menu and select Create from bitmap (Figure 7).
Step 4: Edit the STL File
Choose the image (Figure 8). Choose and set the following parameters according to the end- product dimensions (Figure 8).
Depth (z) – the depth of the gray shade
Depth (a) – the depth of the model base
Step 5: Cut the Unwanted Details
Click on the View menu and choose Front view (Figure 9).
Go to Tool – Cut and choose a circle or polyline cut, depending on the model type (Figure 10).
Remove the excess area (Figure 11).
Step 6: Save
Save the part.