There is no simple answer, the answer depends on the image contents and on the number of unique colors in the image.
For photos jpeg is the best option, compression is lossy, but for the human eye the differences are not that dramatic. You can adjust the compression level (some tools allow it, some not).
For small images, with a maximum of 256 colors .gif or .png are good. Both use loss-less compression.
You can use an image converter to test some of the possibilities (gif, png or jpeg).
By reducing the size if a image
Saving an image in JPEG format results in a smaller file size and some loss of image quality due to compression. On the other hand, saving an image in TIFF format preserves the original image quality but results in a larger file size.
the screen format
the .mpg format or the cam-rip
GIF for images that contain 255 colours or less. Else JPG/PNG.
GIFs are small but poor quality.
Saving an image in the JPEG Fine format offers benefits such as high image quality, smaller file size, and compatibility with most devices and software.
Full form of GIF format of image is Graphics Interchange Format. This format of image can able to save only 256 colors which makes this format poor in the comparison of other image format.
The maximum quality setting for saving an image in the JPEG format is typically 100. This setting preserves the highest level of image detail and results in a larger file size.
To resize an image file size effectively, you can use image editing software to reduce the dimensions of the image, adjust the image quality or compression settings, and save the image in a different file format that is more efficient for web use, such as JPEG.
Pixel Pitch is the measurement of the smallest bit of data in a video image. The smaller the size of the pixels in an image, the greater the resolution.
BMP or PNG of size smaller than 240X320.