Until recently, the photos you cite of Hong Tianguifu and Hong Rengan were up on the two people's Wikipedia pages, but I will confess to having always been somewhat sceptical, and I see that in the months or even years since I last visited those pages, the photos have been removed (which is for the better IMO). As you cite three figures, why not go through them in turn?
Hong Tianguifu was just under 15 when he was executed by the Qing, having nominally occupied the Heavenly Throne for a mere five and a half months – only one of which was spent in the Taiping capital before it fell to the Qing. It is not Hong Tianguifu in the photo linked earlier, and not only because his clothing does not at all resemble that of known Taiping dress styles. A quick search through on Wikimedia Commons shows that users discovered the actual provenance of the photo and that its claim to being of Hong Tianguifu is a hoax. It is in fact part of a wedding photo auctioned in 2015, showing an arranged child marriage during the Republican era, half a century after Hong Tianguifu was executed.
The photograph allegedly of Hong Rengan during his time in Hong Kong has very little provenance attached. In English, it only appears on taipingrebellion.com, a very mixed bag as a reference site, although there are at least some Taiwanese and Chinese websites also allegedly depicting him. In reality though, this is not a photo of Hong Rengan at all, as can be seen from the more high-quality versions of the photo in which the subject can very clearly be seen sporting a thick grey moustache (Hong Rengan was in his 30s during his time in Hong Kong). A reverse image search suggests that the subject of the photo may actually be comprador and reformist Zheng Guanyin, at least according to a page on the English-language website for his birthplace, Zhongshan City. How the photo came to be associated with Hong Rengan is unclear.
Hong Xiuquan was notoriously reclusive after his coronation, and was never photographed during his lifetime. There are, however, two hand-made images that may be contemporary depictions of him. The more well-known is an engraving from Yvan and Callery's sensationalist 1853 work L'Insurrection en Chine, which actually does not purport to depict Hong himself but rather the imagined figure of 'Tièn-Tè', distinct from Hong Xiuquan who was identified as one of his subordinates. However, if it was indeed somehow drawn from life, or derived from an image that was, then it is not impossible that the original subject was indeed Hong Xiuquan. The other, which is quite obscure indeed, is a portrait by the Shanghai painter Tingqua which used to be hosted on an art dealership website but which has since been taken offline (hence the archive.org link). How it came to be painted is unclear, and the colour of the clothing is not what would be expected for Hong Xiuquan, but it is at least more likely to be an authentic rendering. Seemingly contemporary copies of the painting circulate on parts of the Chinese internet, but again, attribution is tricky.
Many members of the Hong clan actually escaped China in the aftermath of the Taiping War, and we actually have some photos of them. One of Hong Rengan's sons, Fung Khui-Syu, fled eventually to Guyana, and there are photos of him such as this one, and ones of his own sons, compiled here. There is also an alleged photograph of Hong Quanfu, a member of the extended Hong clan although his exact relationship to Xiuquan is unknown; he fled to Southeast Asia before quietly cropping up in Hong Kong and getting roped into a plot to establish a second Heavenly Kingdom in Guangzhou in 1903 (which ended in disaster). So we do have some photographic record of the wider clan, but not of its most key members during the period of the original Heavenly Kingdom.
