The entire image just needs to be moved up a pixel either in the C2 image editor or elsewhere. So don't resize or anything for your solution. Select the entire image and move it up so that the bottom pixel row is transparent.
But, more importantly, video cards can only store images in their memory ("vram" or video ram) in power-of-two sizes. This means 2x2, 4x4, 8x8, 16x16, 32x32, 64x64, 128x128, 256x256, 512x512, 1024x1024, 2048x2048 (this is the cutoff for iphone 4 and some low-end gpus), 4096x4096 (this is the cutoff for modern iphones and ipads to my knowledge as well as many gpus), 8192x8192 (absurd, but high-end gpu's can do it.) Any image you make that is above any of those numbers will be rounded up to the next size. So a 520x540 image will be padded out on the graphics card to a 1024x1024 image (with transparent/invisible pixels filling in the rest of the space. This is stored uncompressed like a bitmap on the card. So youre 7,000-something pixel wide image is being loaded as 8,192 pixels wide. If the gpu only can do square images, then 8192x8192. If it can do rectangular power-of-two textures, then you fare a little better.
But the real answer is that your artist needs to be producing art within the constraints of actual game performance. It should never be a 7000-pixel wide image. Your background images are already Tiled Backgrounds (which mean they would repeat their texture instead of stretching/resizing), so your art should be in something like 1024x1024 chunks that can be repeated/looped seamlessly horizontally. This way, you can load that one image and then tile it out across the map. If you want more variation, make a few of them that share a common edge and then you can butt them up side-by-side in addition to having them tile/loop horizontally.