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One color entry in a single GIF or PNG image's palette can be defined as "transparent" rather than an actual color. This means that when the decoder encounters a pixel with this value, it is rendered in the background color of the part of the screen where the image is placed, also if this varies pixel-by-pixel as in the case of a background image.
The only requirement was that this image was invisible, either by being the same color as the page, or by being transparent. Spacer GIFs themselves were small transparent image files. GIF files were used as it was a common format that supported transparency, unlike JPEG. These files were commonly named spacer.gif, transparent.gif or 1x1.gif.
The Graphic Interchange Format (GIF; / ɡ ɪ f / GHIF or / dʒ ɪ f / JIF, see § Pronunciation) is a bitmap image format that was developed by a team at the online services provider CompuServe led by American computer scientist Steve Wilhite and released on June 15, 1987.
Find and select the file or image you'd like to attach. Click Open. The file or image will be attached below the body of the email. If you'd like to insert an image directly into the body of an email, check out the steps in the "Insert images into an email" section of this article.
The PNG file format supports 8-bit (256 colors) paletted images (with optional transparency for all palette colors) and 24-bit truecolor (16 million colors) or 48-bit truecolor with and without alpha channel – while GIF supports only 8-bit palettes with a single transparent color.
Animated Portable Network Graphics (APNG) is a file format which extends the Portable Network Graphics (PNG) specification to permit animated images that work similarly to animated GIF files, while supporting 24 or 48-bit images and full alpha transparency not available for GIFs.
When converting an image from the PNG format to GIF, the image quality may suffer due to posterization if the PNG image has more than 256 colors. GIF intrinsically supports animated images. PNG supports animation only via unofficial extensions (see the section on animation, above). PNG images are less widely supported by older browsers.
If you've just passed an image challenge, but are immediately prompted by another, there may be an issue with your browser. Before you attempt another image challenge, clear the cache on your web browser. If you're still prompted to retake the image challenge multiple times a day, you might have a more serious issue.