There is no golden solution for image files. You should generally save them in the highest possible resolution and with the most lossless compression possible. This makes little sense, however, if the image files are already compressed. Our tips and recommendations:

• TIFF: Classic long-term preservation format, however only uncompressed; as TIFF 6.0, Part 1: Baseline TIFF. There is no guarantee that other TIFFs are suitable for long-term preservation.
• JPEG2000 and PNG: Both recommended for lossless compression. JPEG2000 is also used in PDF/A-2.
• jpg: Perfectly acceptable for many pre-existing files in this format because it is so widespread.
• To be avoided: Unusual and proprietary formats, including RAW, Photoshop PSD, PCD, Encapsulated PostScript, FlashPix.
• If you have any questions about other file formats, please get in touch.

Tip: There are several graphics programs available, e.g. Irfanview, that allow you to convert a batch of multiple files at the same time.