Can I use Photoshop 5.0 on a iMac system 10.10.6

No.

I believe Photoshop 5 is pre-Intel software that will only run on older PowerPC Macs running on older pre-OS X operating systems, like MacOS 7, 8 or 9 OR very old versions of OS X that supported Classic mode.

That software is about 20 years old.

That will NOT run on any modern Macs from 2006 and newer.

You will need a more up to date version of Photoshop or a Photoshop equivalent.

New or newer (older Adobe Photoshop CS versions) are very expensive and hard to obtain now.

Adobe charges a monthly subscription fee to use their current Adobe Creative Cloud apps.

My Recommendations

The current lesser Adobe Photoshop Elements

Pixelmator

Affinity Photo

Pixelmator is a very inexpensive and nearly a Photoshop clone in look and feel.

http://www.pixelmator.com/mac/

Affinity Photo is, relatively, new but packs a lot of very impressive features at another inexpensive price level.

https://affinity.serif.com/en-gb/photo/

Back when, Adobe sold Windows and Mac versions of their software separately.

No Mac/PC discs in one software package back then.

This version of PS is from the 1990's.

If the OP has the disc coded for the Mac version for PowerPC Macs running System 7.5, and MacOS 8 and 9 (again, 1990's Mac OSes) and may ONLY run on early versions of Mac OS X that supported Classic MacOS 9 mode (Classic mode was only supported up to OS X 10.4.11 Tiger).

If the OP has the Windows version,

The Windows version might be viable on the Windows side of Mac, using BootCamp OR vitualization provided that today's Macs can install and run Windows 95 or NT 4.

Still, we're talking over 20 year old Windows software.

May still be a crawpshoot on the Windows side of things on the Mac, no?

It was doubtful to work, anyways.

Photoshop 5, 5.5 are 20 year old software.

Photoshop 6 and 7 are just as old!

Do YOU really think that Photoshop 5 was going to run on a modern day Mac with or without installing Windows?

Me thinks not!

My advice/opinion stands.

Actually, according to the Photoshop website at Older versions of Photoshop system requirements

Photoshop 5 will work on Pentium or faster Intel processer in Windows, and that would cross over to Mac system requirements at that time:

• Pentium or faster Intel processor
• Windows 95 or Windows NT 4.0 or later
• 32 MB of RAM (64 MB recommended)
• 80 MB of available hard-disk space
• 256-color (8-bit) display adapter (24-bit color recommended)
• CD-ROM drive
• Sound card recommended for viewing interactive tutorial files

Mac OS

• PowerPC based Macintosh computer
• Apple System Software 7.5 or later
• 32 MB of RAM (64 MB recommended)
• 80 MB of available hard-disk space
• Color monitor with 256-color (8-bit) or greater video card (24-bit color recommended)
• CD-ROM drive

Yes. From the Photoshop website at Older versions of Photoshop system requirements

• PowerPC based Macintosh computer
• Apple System Software 7.5 or later
• 32 MB of RAM (64 MB recommended)
• 80 MB of available hard-disk space
• Color monitor with 256-color (8-bit) or greater video card (24-bit color recommended)
• CD-ROM drive

*sigh*

The sad part is, the OP will probably never bother installing it and seeing that it does work.

Might be a good idea to not talk folks out of trying stuff because you think it won't work.

At the end of the day, I provided documentation that it will work. I'll take my documentation over your opinion.