It's hard to say where the problem lies for sure and it's been a long time since I worked with GA 6.2 (I currently run two seats of Omega 2.0.2). Here are some things to consider or to experiment with:
1. Make note of the actual pixel dimensions rather than the size in inches of your image. Most Edge users agree that the ideal size for an image is 100 pixels per inch of output dimension. So if you are doing a fifty inch print then you should create an image with pixel dimensions of 5000. More is overkill and GA 6.2 is a 16 bit application with some limitations on how large a file it can handle. Omega is 32 bit and much quicker and more powerful. What pixel dimensions are you working at in Fireworks?
2. My recollection of GA 6.2 is that it always imported the image at its true size at 300 dpi so it always needed resizing if it wasn't sized and saved at 300 dpi. In other words, as in the above example, a 5000 pixel image at 100 dpi would import as an image that only measured 16.667".
3. Files types and color modes ..... If you are saving TIF images, I recommend that you have that image in a color mode setting of CMYK. If you are saving as JPG, then use a color mode setting of RGB. When the image is imported into Composer, it is interpreted into CMYK but with different setting than were originally set in your image editor. This is necessary because thermal resins such as are used with the Edge require different percentages to achieve the same color as compared to an inkjet. I prefer JPG because it is RGB and the Gerber interpretation of RGB to CMYK is superior to Gerber's interpretation of CMYK to CMYK. But I digress .... main point is use the correct color mode for the file type you are saving in your image editor.
4. Composer does not show you the actual image in its workspace. For whatever the reason, supposedly to save on file size, a thumbnail representation of the image is used. The controls for this are located in the Image Fill Dialog. There is a control for Thumbnail Bits per Pixel and another for Thumbnail Resolution. (See screen shots.) A setting on each slider farthest to the right will provide you the best thumbnail in Composer but at a sacrifice of a larger saved file size. The underlying image, however, is what is used to print with so the question becomes: Are you being fooled by the low rez thumbnail into thinking your image isn't any good?
All that having been said, if you want to send me an image file that is known to be giving you a problem, I'll be happy to analyze what the problem is and provide you with feedback and solutions. Just attach to an email and send it to the email address in my signature.