Are there any .NET libraries etc. which allow adding layers (with text) to .tiff images? Something like annotations, so that it would be possible to separate them from image afterwards.
I tried DotImage - it allows to add annotations, save them as embedded into image and load them afterwards, but no one other image viewer seems to recognize that they are there.
The TIFF format allows you to add add/embed anything in the file: that implies that there is no TIFF viewer which knows how to display everything which could be embedded in a TIFF file.
I work for Atalasoft and have worked on DotAnnotate. The reason why there is no interoperability between, say, Xmp formatted annotations embedded in a TIFF from DotAnnotate and IrfanView is that our annotations are a proprietary, but specified format. If you want a viewer to read them, you would need tool to convert them (which can be built with our own assemblies).
TIFF by itself doesn't specify an annotation format. It does specify ways to embed non-image data into a TIFF file in a way that it can be pulled out. In most cases, TIFF neither dictates what the data should be nor how it should be formatted.
PDF, on the other hand, does define how certain types of annotations should appear, behave and be stored in the the document. PDF annotations and DotAnnotate annotations intersect in capability, but not 100%. There are classes of PDF annotations that are not (yet) supported in DotImage and vice versa. There are also core capabilities of DotAnnotate annotations that can't be represented (well, if at all) in PDF.
DotAnnotate, however, does have the ability to round-trip annotations in PDF by creating a facade annotation (usually a rectangle annotation with an appearance stream that makes it look like the DotAnnotate annotation) and embedding the annotation data into the PDF annotation.
If you want to store annotations in a TIFF and offer a web view, you can do that with our Ajax annotation viewer, which you can try out here.
If there is a specific goal or use case you need to solve, you should absolutely contact support directly for help.