An image size of 1280 x 720 pixels is large enough to be HD standard and is commonly used in photography and film. This size uses a 16:9 aspect ratio, which is also common in filmmaking.
The most popular sizes for photo prints are: 4×6: 4×6 prints measure approximately 4” x 5 ⅞”. This is the standard size in the photofinishing industry because this print size mirrors the aspect ratio of most digital cameras viewfinder.
2400x1600px, jpeg, saved for web, and optimized
To ensure that your full width images look good across any device big or small the recommended size is 2400x1600px. Note that devices have different ratio than the one you shoot and it is possible your images will be cropped when viewed on web.
Photo Printing：4 x 6 or 5 x 7 inches is the most standard and popular photo size, usually used for display artistic photos or greeting card printing.
Average image size per format: JPG: 11.8 KB, PNG: 4.4 KB, GIF: 2.4 KB. There are 42.8 images per Web page on average. Images contribute to 61.3% of the download size of a Web page on average. There is an average of 240 KB of images on each web page.
There are actually no differences between the JPG and JPEG formats. The only difference is the number of characters used. JPG only exists because they required a three-letter extension for the file names in earlier versions of Windows (MS-DOS 8.3 and FAT-16 file systems).
TIFF — Tagged Image File Format
TIFF is the workhorse format for high-resolution printing, including art photography and other fine artwork. It's a lossless image file format that retains extremely high photographic quality for scanning and printing.
For regular photos, then 3:2 or 4:3 would suffice. When choosing a format, think about the type of camera you have. To maintain quality, the ratio of your image size should never exceed your camera sensor's. For instance, if you're using a micro four-thirds camera, stick to 4:3 or 1:1.
RAW files are the highest quality image format. They are loved by photographers as RAW format records all data from the sensor of the camera. Since RAW is an uncompressed format, it gives immense creative liberty to the photographers during post-processing.
Along with RAW, TIFF files are among the highest quality graphic formats available. If you're printing photos—especially at enormous sizes—use this format. You are making a high-quality scan. Using TIFF to scan your documents, photos and artwork will ensure that you have the best original file to work off of.
PNG is a high-quality graphics format – generally higher in quality than JPEGs, which are compressed to save space. The PNG format uses lossless compression and is generally considered a replacement to the Graphics Interchange Format (GIF format).
PNG does not degrade image quality, but the files are larger. The JPEG standard was invented for situations where file size is critical and image quality is not important. Use PNG when image quality is critical and file size is not important.
jpg file format has lossy compression. Since these two formats have differences, it is often necessary to turn PNG to JPG or vice versa. Lossless or lossy compression? PNG files are in nature lossless which means the quality of the image stays almost unalterable when the file is compressed.
JPEGs is perhaps the most common file format for printing digital photographs. Most photographs taken on both cameras and cellphones are automatically saved in JPEG format. JPEGs are high-quality, and they have very manageable file sizes, giving them a big advantage over other image formats.
jpg. This most used image file format is used to store photos all over the world, and is generally a default file format for saving images.
The TIFF prevents viruses from hiding in its image. The TIFF image type prevents users from placing links on the images, which would negate the file's privacy. For example, when users click a link on an image they expose things like their location through their IP sometimes.
The most common aspect ratios for standard photography and art prints are 3:2 and 5:4.
I'd suggest you give them full size photos. Currently, when I export my photos in Lightroom to deliver to a client, I do not resize the photos down, I set “quality” to 100, and set the resolution to 300 ppi (in case they want to print the photos).
The aspect ratio for photos is the proportional relationship between the width and height of an image. The most common aspect ratios are 4:3, 3:2, and 16:9.