System requirements

Windows: .NET Framework 4.5 (comes with Windows 8, should be installed manually on Vista and 7), or mono.

Linux, Mac OS X, FreeBSD: mono (earliest tested version is 5.10). If "mono-complete" package is available for your OS, install it.

Changelog

v.1.7:

Changed the _thumb rename rule. Added -imgnum parameter.

v.1.6:

Added support for downloading custom background images.

v.1.5:

Added command line parameters for easier use of fantiaimgs in batch/shell scripts and for a more precise behavior control (most notably in renaming output files).

Command line arguments

Argument	Description
-h, -help	Prints supported command line arguments and exits.
-club 123	Digital ID of the fanclub to dump. For example, "11791" for https://fantia.jp/fanclubs/11791. If not specified, user will be prompted to enter it manually.
-name ABC	Specifies the name of the folder to dump data into. To avoid issues prefer to use short ASCII filenames. If not specified, user will be prompted unless "-club" argument is also present, in which case club ID will also be used as a folder name.
-cookiefile FILE	Tells fantiaimgs to look for a _session_id cookie value in the specified FILE. May be an absolute path or relative to the current working directory. If not specified, "_session_id.txt" file is assumed. If specified file does not exist, user will be prompted to enter that cookie value.
-subp	Prepends an index of subpost within a post to the filenames. Helpful to avoid most naming conflicts when used together with "-nh".
-imgnum	Prepends an index of an image within a post to the filenames. When used together with "-subp", subpost index goes first.
-nh	Removes a small hashsum in the beginning of filenames, i.e 1234abcd_File.png -> File.png Note that it may cause name conflicts, especially when used without "-subp". In this case, a "+" character will be prepended to the newer conflicting name.
-keepthumbnames	By default fantiaimgs will rename post thumbnail pictures to "_thumb" starting with v.1.5Changed in 1.7. If this is not a desired behavior, use this switch. In versions prior to 1.7, this feature used a filename template on all pictures in post, which could misinterpret non-thumb pictures as such.
-nometa	If specified, header image, fanclub icon and the custom background image will not be downloaded.

When not specified otherwise explicitly, arguments listed above are applicable to the latest stable version.

FAQ

Is this a free software?
Yes. You can find the source code on Github.
Will this tool let me download paid content for free?
No. That's why you need a valid cookie to use it. For example, if you provide a cookie given out to guests, you'll only be able to download content visible to guests. Note that most fanclubs make a lot more content available to a 0 yen level subscriber than to an unregistered user, so having an account is worth it even if you don't intend to pay anyone.
How can I suggest a feature or report a bug?
One option is to DM oxi98144#5772 on Discord, another one is to post an issue on the Github page