Hi Eric,
The phones have different resolutions depending in which hardware version you use:
X7 = 800x400
X7C = 800x400
X7-V2 = 1024x600
X7C-V2 = 1024x600
Also keep in mind that while the phones have a specific supported resolution, some models might be smart enough to resize it to their needs if you provide a different size image, while others will not. Best to look up the manufacturer documentation if you want to double-check, based on the hardware revision. Some models also need you to encode the image in a RGB565 or RGB888 encoding format (yep, there are different types of bmp). Personally I also like to download a screenshot of the LCD from the phone webUI menu whenever I want to verify.
Hope this info is useful to you!