The recipe recipes-multimedia/libpng/libpng_1.6.24.bb uses a URL that does not seem to be available anymore. The version libpng-1.6.24.tar.xz seems to have been deprecated (not present on the server anymore), so the recipe should either use different URL or be upgraded to 1.6.29, which is the latest as of now.
ERROR: libpng-1.6.24-r0 do_checkuri: Fetcher failure for URL: 'http://distfiles.gentoo.org/distfiles/libpng-1.6.24.tar.xz'. URL http://distfiles.gentoo.org/distfiles/libpng-1.6.24.tar.xz doesn't work ERROR: libpng-1.6.24-r0 do_checkuri: Function failed: do_checkuri ERROR: Logfile of failure stored in: /home/pokybuild/yocto-autobuilder/yocto-worker/nightly-checkuri/build/build/tmp/work/i586-poky-linux/libpng/1.6.24-r0/temp/log.do_checkuri.16305 NOTE: recipe libpng-1.6.24-r0: task do_checkuri: Failed ERROR: Task (/home/pokybuild/yocto-autobuilder/yocto-worker/nightly-checkuri/build/meta/recipes-multimedia/libpng/libpng_1.6.24.bb:do_checkuri) failed with exit code '1'
Punting these to Maxin: I think you moved libpng to gentoo download site at some point?
http://lists.openembedded.org/pipermail/openembedded-core/2017-July/139886.html
Same failure seen with https://autobuilder.yocto.io/builders/nightly-checkuri/builds/396 morty 2a70e84643381eca0e7bf7928d4a3d56f9651128. Log attached.
Created attachment 3981 [details] log of failed build #396 on morty
Pyro is having same issue but on libpng-1.6.28 https://autobuilder.yocto.io/builders/nightly-checkuri/builds/454/steps/BuildImages/logs/stdio
looking at all this checkuri issue for stable branches
http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?h=pyro&id=cc9ab83f7fa390e42742e18efe56009146c2de0b
in pyro