If you have an image that you don't want to automatically by resized on upload to Mitingu, then it's a good idea to change the canvas size. This basically means the background area that the image sits within.
No need to have a Photoshop licence here, just use a free online editing tool like pixlr
1. Open up your image in pixlr, then select Image/Canvas Size
2. Key in the required canvas size (1400 x 300px for the Basic Theme and 1400 x 600px for the Stylish Theme) and select the anchor point (do you want the image in the centre, left, right etc)
3. Save your file as a .png or .jpg and then the image will sit in the banner at required size