If you are using our system templates you will see that there are clearly defined image boxes just waiting for you to insert your own images.
A: In order to get the best fit you need to know first the size of the image box in the template. To do this simply right-click over the image box and find the size in pixels e.g. If you're using Firefox browser then right click on the image and choose inspect element; if you're using Chrome then right click and choose inspect element as-well and scroll down on the right side of your screen.
Once you know the size in pixels e.g. 555 x 217px then you can check your own image you are wanting to bring in.
You may need to re-size your image which you can do easily in programs like Paint. In Paint for example just load your image, choose pixels and if you maintain aspect ratio then you can just change the horizontal and the vertical will change in ratio automatically. However you may wish to untick the "maintain aspect ratio" and then change the horizontal and vertical manually and independent of one another.
If your image is only marginally larger than the destination image box, then you should be fine with most of our templates, as the image boxes are robust to contain these.
You can then send yourself a test to see your draft results before you send out.
Hint: If you are re-sizing your image, make sure to re-save it before you use it in your campaign!