How to reference URL relative to current page in a block
Adding a relative path (without the starting "/") is WRT the current "directory", so when you're on a node page the "directory" is "node" and the NID is the "page". Your "images" is supposed to be another page within the same "directory". That's the simplified explanations about how relative paths work in general. 

To solve your problem the easiest way is probably through a Token. Do you have Token Filter or a similar module installed and configured? If your Text format doesn't evaluate tokens you won't get the correct output. 

Try some available tokens in your text and see what they output. Maybe there's a [current:path] or similar token, or [current:nid]. When you find one with the output you like you can contruct the link: 

&lt;a href="/node/[current:nid]/images"&gt;Images&lt;/a&gt;

or 

&lt;a href="/[current:path]/images"&gt;Images&lt;/a&gt;

If you need more control over the Block Visibility you can try Block Exclude Pages. 

Another way to create contextually aware links is through Views (Block display). You would have a Node ID as the contextual filter, and override the Link to content field to add "/images" at the end of it.