To get the best image quality on your phone you should try to use high resolution images in the dimensions best described to fit your phone set.
If you are using a Mitel 5360 images 480 pixels wide x 800 pixels high are recommended.
If you are using a Mitel 5320/5330/5340 images 160 pixels wide x 320 pixels high are recommended.
You may have to edit the Html in blogger after you have uploaded your photo to ensure that you are getting the proper sizing. All you will have to do is edit the width and height of the image.
The areas that are highlighted in the image below indicate where you will need to make your edits:
