If Light Image Resizer downloaded from ObviousIdea's website is flagged as a virus or considered unsafe by your anti-virus software, this is most likely a false positive.
It's a different case if you download it somewhere else, so to be safe you should only use the official download.
If you want to be sure, you can submit the file to online virus scanners, where it will be checked by a number of different anti-virus products:
In any case please contact us, and mention which antivirus product you are using. In addition you can submit the file to your anti-virus vendor, so they will analyse and whitelist it if needed. Some even allow to do this in the warning notice (see "Details" or "More options", "Report false positive").
False positives should be reported to the respective security vendors:
VirusTotal - False Positive Contacts
Related:
Our installer is flagged as WS.Reputation.1? [Click here for more information]
Light Image Resizer flagged as malware, virus or trojan
Moderator: Fabrice