No friends yet.
I guess you mean Elgg version 2.3.1.
Tidypics version 2.0.0 or newer should work fine on Elgg 2.0 - 2.3. For Elgg 2.3 I recommend using Tidypics version 2.3.3.
Are you sure that you have installed Elgg version 2.3.1? Is it a fresh install or have you updated from an older version of Elgg? In the latter case have you completed the upgrade by running the database update that is the last step when updating the Elgg version used (either by clicking on the "Upgrade" button in the Elgg admin section or by calling your.site.url/upgrade.php in your browser)? If you updated from a version older than 2.3.0 and you haven't run the database upgrade the Elgg version returned when checking for the Elgg version installed will still be the old version. And then Tidypics version 2.3.3 can't be activated because it requires Elgg 2.3.0 at minimum.
You can also check if all dependencies of Tidypics are fullfilled by clicking on the "Tidypics" link in the plugin list entry of Tidypics. If a dependency is not fullfilled, it will show up there (though the only depencency is the version of Elgg used).
Or the reason for Tidypics not getting activated might be either the Tidypics plugin folder being incomplete (error on upload) or some files of Tidypics having wrong permissions/owner (changed due to the way you uploaded Tidypics. You can try to extract the Tidypics plugin folder directly in the mod directory of your Elgg installation on the server. Then is quite unlikely that the installation is incomplete or there's a permission issue.
I can't think of anything else that might prevent Tidypics from being enabled at the moment. If the Elgg version is not the problem and the files of Tidypics neither, there might be some error logged in the server logs explaining what's wrong. But I can say for sure that Tidypics version 2.3.3 should definitely work on Elgg 2.3.