ACA WebThumb ActiveX Control takes the snapshots of given URLs as BMP, JPG, GIF, PNG, TIFF, WMF and EMF image. These are not the only choices, but they are good and reasonable choices.
Web pages require JPG or GIF or PNG image types, because that is all that browsers can show. On the web, JPG is the best choice (smallest file) for photo images, PNG is most common for screenshot images, and GIF is most common for graphic images.
Other than the web, TIF file format is the undisputed leader when best quality is required, and TIF is very common in commercial printing or professional environments. High Quality JPG can be pretty good too, but don't ruin them by making them too small. Try to make them large instead.
best image quality, large file
To get best image quality, you should choose BMP file format(Windows Bitmap). It's used internally in the Microsoft Windows operating system to handle graphics images. These files are typically not compressed, resulting in large file.
Lossy format, small file
JPEG(Joint Photographic Experts Group) files are a lossy format (in most cases). The DOS filename extension is JPG, although other operating systems may use JPEG. Nearly all digital cameras have the option to save images in JPEG format. The JPEG format supports 8 bits per color - red, green, and blue, for 24-bit total - and produces relatively small file sizes. The compression when not too severe does not detract noticeably from the image. But JPEG files can suffer generational degradation when repeatedly edited and saved. Photographic images may be better stored in a lossless non-JPEG format if they will be re-edited in future, or if the presence of small "artifacts" (blemishes), due to the nature of the JPEG compression algorithm, is unacceptable. JPEG is also used as the image compression algorithm in many Adobe PDF file.
Limited color, Lossy format, small file
GIF(Graphics Interchange Format) is limited to an 8-bit palette, or 256 colors. This makes the GIF format suitable for storing graphics with relatively few colors such as simple diagrams, shapes, logos and cartoon style images. The GIF format supports animation and is still widely used to provide image animation effects. It also uses a lossless compression that is more effective when large areas have a single color, and ineffective for detailed images or dithered images.
True color, small file
PNG(Portable Network Graphics) file format is regarded, and was made, as the free and open-source successor to the GIF file format. The PNG file format supports true color(16 million colors) whereas the GIF file format only allows 256 colors. PNG excels when the image has large areas of uniform color. The lossless PNG format is best suited for editing pictures, and the lossy formats like JPG are best for final distribution of photographic-type images because of smaller file size. Many older browsers do not yet support the PNG file format, however with the release of Internet Explorer 7 all popular modern browsers fully support PNG.
Widely accepted in printing industry
TIFF(Tagged Image File Format) is a flexible image format that normally saves 8 or 16 bits per color - red, green and blue - for a total of 24 or 48 bits, and uses a filename extension of TIFF or TIF. TIFF's flexibility is both a feature and a curse, with no single reader capable of handling all the different varieties of TIFF files. TIFF can be lossy or lossless. Some types of TIFF files offer relatively good lossless compression for bi-level (black and white, no grey) images. Some high-end digital cameras have the option to save images in the TIFF format, using the LZW compression algorithm for lossless storage. The TIFF image format is not widely supported by web browsers. TIFF is still widely accepted as a photograph file standard in the printing industry. TIFF is capable of handling device-specific color spaces, such as the CMYK defined by a particular set of printing press inks.
Windows Metafile(WMF) is a graphics file format on Microsoft Windows systems, originally designed in the early 1990s. Windows Metafiles are intended to be portable between applications and may contain both vector and bitmap components. In contrast to raster formats such as JPEG and GIF which are used to store bitmap graphics such as photographs, scans and graphics, Windows Metafiles generally are used to store line-art, illustrations and content created in drawing or presentation applications.
WMF is a 16-bit format introduced in Windows 3.0. It is the native vector format for Microsoft Office applications such as Word, PowerPoint, and Publisher. a newer 32-bit version with additional commands is called Enhanced Metafile(EMF). EMF is also used as a graphics language for printer drivers.