Pixcavator can handle these image formats: JPEG, BMP, PNG, GIF, TIFF.
It's always better to choose the original format if possible. Adobe Photoshop, MS Paint can convert formats. But, converting to such a "lossy" format as jpeg will degrade your image. For example, particles separated by thin gaps will merge.
It's always better to choose the original size if practical. Once again, reducing the size (and resolution) of the image will degrade it -- particles separated by thin gaps will merge. (Keep in mind that anything larger than 1000x1000 would make you wait).