Photo resolution matters on the web

72 dpi resolution

You can use on your website, on your WordPress blog jpeg, gif, png, or bmp image graphic files.
They are all different than each other how they compressed and their image quality.
If you are going to use an image on your website you need to pay attention the image resolution.

Images come from a digital camera are usually JPG files. JPEG file format supports 16 million colors. JPEG format supports 8 bits per color (red, green, blue) for a 24-bit total, producing relatively small files.
GIF format only supports 256 colors. GIF only allows 8-bit images. They allow transparency and animation.
TIFF files are flexible format that normally save 8 bits or 16 bits per color (red, green, blue) for 24-bit and 48-bit totals.They are good for printing not for web displaying.
PNG files support true color (16 million colors) while the GIF supports only 256 colors. So they are large file size than gif files.
Images come from a scanner are actually bitmap files. They are good for printing not for web viewing. Windows bitmap files are stored in a device-independent bitmap (DIB) format that allows Windows to display the bitmap on any type of display device. The
term “device independent” means that the bitmap specifies pixel color in a form independent of the method used by a display to represent color.

Here is example file size for a photograph.

This photo saved as jpg 72 dpi resolution and file size: 237 kb ( edited with Photoshop )

Same image saved as gif the file size: 98kb

Same image saved as png the file size: 270kb

Same image saved as tif the file size: 478kb

Same image saved as bmp the file size: 542kb

500px width, jpg file = 237kb
File Extension:

.gif Graphics Interchange Format graphic, used primarily on the Web
.png Portable Network Format graphic used primarily on the Web
.jpg or .jpeg Joint Photographic Expert Group graphic used primarily on the Web

Commonly used for printing:

.bmp Microsoft bitmapped file
.pdf Portable Document File
.eps Encapsulated Postscript File (graphic)
.ai Adobe Illustrator
.tif/.tiff Tagged Image File Format graphic file
.psd Adobe Photoshop Document
.svg Scalable Vector Graphics File

500px width, gif  file = 98kb